1. Field of the Invention
This invention relates to a rack shaft supporting device for rack-pinion steering of a vehicle, and more particularly to a rack shaft supporting device in a rack-pinion type steering apparatus used as the steering apparatus of a passenger car or the like. The device supports a rack shaft by antifriction support means capable of adjusting the load bearing area and subjected to a friction force and includes limiting means for limiting the displacement of the rack shaft over a predetermined amount.
2. Related Background Art
The rack-pinion type steering apparatus is widely used in FF type vehicles because it is simple in structure, inexpensive, and precise in steering as compared with the ball-screw type steering apparatus heretofore universally used.
However, as the utilization of the rack-pinion type steering apparatus has spread, problems have come to be recognized in connection with the rack shaft supporting device.
Generally, in order to prevent back-lash from occurring in the mesh engagement between the rack and the pinion, a rack shaft supporting device which urges the rack shaft toward the pinion is provided on the rack at a position opposed to the pinion. One such device which uses a sliding support is popular, but one utilizing an antifriction support has also been studied.
In a rack shaft supporting device of the type in which a shaft supporting member is urged toward the pinion by a spring and the rack shaft is sliding-supported by a bearing surface, the rack shaft is always surface-supported, and therefore, a great steering force is necessary.
On the other hand, in a rack shaft supporting device of the type in which a rolling antifriction member is utilized to support the rack shaft, the antifriction member is subjected to the severe shock from the road surface and therefore, it is necessary to reinforce the rolling portion. In addition, the support of the rack shaft is point-support or line-support, and this has led to a problem in that minute swivelling vibration (hereinafter referred to as "shimmy") of wheels is liable to occur during high-speed driving.
Further, according to the latest studies, a two-phase type rack shaft supporting device is known which uses both the sliding support and the antifriction support.
For example, a two-phase type rack shaft supporting device has been proposed which effects sliding support in low load conditions and additionally effects antifriction support in a special high load condition. In this case, the sliding support is used in normal conditions and is supplemented by the antifriction support in a very high load condition. Therefore, the steering force is not reduced, and it is necessary to reinforce the antifriction support portion for an excessively great load. This leads to a problem in that a compact construction is difficult to achieve.
Also, a two-phase type rack shaft supporting device has been proposed which utilizes antifriction support in low load conditions and sliding support in high load conditions. However, in such a rack shaft supporting device, the rack shaft is supported only by antifriction support in the low load conditions such as high-speed driving. Therefore, the support becomes point-support or line-support, and the absorption of shimmy is liable to become insufficient.